The conventional method of dispersing gases into a liquid is to use a mixing apparatus including a vessel for the liquid, a rotating radial flow impeller immersed in the liquid with its axis vertically oriented, and a gas distribution jet or header in the vessel under the impeller. The impeller or radial flow turbine thus disperses the gas introduced into the liquid via the gas jet means. When the blades of the turbine are rotated in the liquid, the hydrostatic pressure in front of the blades increases and decreases behind the blades. This is a natural consequence of the hdyrodynamic resistance which, together with the centrifugal and Coriolis forces urge the fluid in a radial direction. However, the pressure difference results in that the gas bubbles move to the low pressure areas behind the blades, where they collect and combine into larger gas cavities. In practice, these cavities result in a streamline forming of the blades, which signifies a drastic reduction of the hydrodynamic resistance, and thus also a drastic reduction of the power required to rotate the turbine. In order to retain a desired degree of agitation, it is therefore necessary to install a very much greater and thus more costly agitator than would otherwise be required. In addition, dispersion of the gas in the liquid is made more difficult by the mentioned coalescing of the gas bubbles and the formation of larger gas volumes on the trailing sides of the blades.
The case may also be conceived where a liquid that is to be mixed contains dissolved gases which it is desired to remain dissolved in the liquid. It may then happen that these gases depart from the liquid due to the low pressure regions behind the blades, forming gas cavities behind the blades, and gradually departing from the liquid in the form of large gas bubbles. The pressure on the trailing surfaces of the blades may also be so low that the liquid is vapourized and the genreated vapour forms the mentioned gas cavities so that in practice these cavities drastically reduce the driving power of the turbine.
A first object of the invention is therefore to provide a blade configuration for a turbine or impeller of the indicated kind, such that the driving power of the impeller does not fall due to the occurrence of such gas cavities on the trailing sides of the blades during operation of the apparatus, particularly in connection with the dispersion of gas into the liquid.